1. Technical Field
The present disclosure generally relates to amplifiers and, more specifically, to audio amplifiers. The present invention more specifically applies to the forming of an output stage of such an amplifier.
2. Description of the Art
Many electronic circuits performing audio amplifier functions are known. Such amplifiers are generally formed of several successive stages between an input stage to which is applied a signal to be amplified and an output stage having an output terminal providing an electric signal to a loudspeaker. The different amplifications stages are generally differential, the output stage being a stage with differential inputs and with a non-differential output.
A recurring problem of amplifiers is the consumption and the loss in output stage transistors due to the low useful signal level as compared with the power supply voltage. This problem is particularly critical in portable devices powered by a battery. It must be possible to decrease the power supply voltage in the idle state or when the amplitude of the signal to be amplified is low.
It has already been provided to use a switched-mode power supply enabling to decrease the output stage power supply voltage according to the level required for the output signal. In this type of circuit, the power supply voltage of the series-connected transistors of the output stage is variable and regulated according to the needs associated with the input signal level.
The minimum value of this voltage must allow a proper operation of the series transistors of the output stage. Even when MOS transistors with a low threshold voltage are used, the efficiency remains unsatisfactory.
To improve the operation of these amplifiers, and especially their harmonic distortion rate, systems adjust the current in the output stage MOS transistors. To achieve this, their gate voltage is controlled according to the result of a measurement of the current that they conduct. Such systems enable to set a current to a value sufficient for the output stage operation, which is the minimum current of the output stage transistors when the output voltage level is zero (idle amplifier).
The output stages to which the present invention applies are so-called push-pull stages, generally formed of two MOS transistors in series between the two power supply voltages, the output terminal corresponding to their junction point. It is then desirable, even in operation, for that of the transistors which is not used to amplify the signal to be crossed by a minimum current.
U.S. Pat. No. 6,366,165 describes an amplifier using the same power supply voltages for the output stage and the control of the gate voltages of the output stage transistors. This requires a power supply voltage greater than a MOS transistor threshold voltage, which decreases the efficiency for low-amplitude signals.
U.S. Pat. No. 5,055,797 describes a solution wherein the respective biasings of the two N-channel MOS transistors of an output stage are controlled based on a measurement of the currents in each of the transistors. This measurement is performed with current-to-voltage conversion resistors interposed in the series association of the output transistors. This measurement is then exploited to control the biasing of the gates of these transistors and, thereby, their respective currents. The presence of such resistors in series with the output transistors introduces additional losses. Further, the values of these resistors are not negligible, since they would otherwise generate an unacceptable inaccuracy of the measurements.
It has already been considered to provide measurement transistors (for example, in U.S. Pat. No. 4,853,645), but the minimum output stage power supply voltage remains significant.
Other circuits set the gate-source voltage of the output transistors in fixed manner (with no control). Such is for example the case for the solution provided in U.S. Pat. No. 5,783,970.
All these solutions are sensitive to temperature variations and to possible mismatches of the transistors.